As many as are needed. A class is a pattern that can be followed an unlimited number of times.
The picture shows a parent class and a child class, and some objects that have been constructed from each. These objects are shown as rectangles (to convey the idea that they are more real than the classes, which are only designs).
In the picture, "Joe's car," "Mary's Ford," and "Bob's Ford" represent objects. The cloudy classes represent designs, not objects.
Objects are constructed as a program runs. An object is constructed by following a description in a class file that was created by compiling a Java source program.